Blown fuse indicator



June 29, 1961 s, w so 2,989,739

BLOWN FUSE INDICATOR Filed Aug. 14, 1957 FIG. 20

INVENTOR.

- +5Ov EDWARD 3. WILSON MWLM ATTORNEY 2,989,739 BLOWN FUSE INDICATOREdward S. Wilson, Poughkeepsie, N.Y., assignor to International BusinessMachines Corporation, New York, N.Y., a corporation of New York FiledAug. 14,1957, Ser. No. 678,211 5 Claims. (Cl. 340-250) This inventionrelates to electrical apparatus and more particularly to means forindicating the occurrence of a blown fuse in low voltage electricalcircuits.

It is common practice to fuse electrical circuits for protection againstfaults which cause the excessive flow of current, and the electricalfuse commonly used is merely a conductor having a section which meltswhen excessive current is drawn through the fuse when a fault occurs. Inmany of the fuses in use today, excessive current causes a section ofthe fuse to melt away which, in turn, permits a movable mechanical linkto operate a fault indicator circuit which may be visual or an audiblealarm. One way of indicating that a fuse has blown is by connecting aglow lamp and a series resistor in shunt with the fuse whereupon openingof the fuse circuit permits firing potential to be developed across theneon bulb, and the glow once struck is an indication that the particularfuse is now open. This method is unsatisfactory in low voltage circuits,for example, those used with transistors where the total voltage of thecircuit is less than the firing potential of the glow lamp. In otherdevices of like purpose, the blowing of the fuse frees a mechanicalmember which closes another circuit thereby providing some indicatingdevice such as a relay coil or a lamp with a suitable voltage. Suchcircuits are adaptable to low voltage applications but areunsatisfactory nevertheless in many instances due to failure of themechanical element to move, or once moved, the failure of the mechanicalelement to provide the contact for completing the indicating circuit.Accordingly, it is the object of the invention to provide an improvedfault indicating apparatus.

Another object of this invention is to provide an improved andinexpensive fault indicator circuit for use with a plurality of lowvoltage sources.

It is a further object of this invention to provide an improved fusedevice which employs a moving link member to indicate the occurrence ofa fault in a circuit.

It is a still further object of this invention to provide an improvedfuse having a movable element which is a current carrying element whenthe fuse is not blown.

It is an object of this invention to provide an improved indicatingcircuit in which the indicating circuit is isolated from the workcircuit by the melting of the fuse element.

It is another object of this invention to provide a fault \indicatingdevice which is reliable in operation and does not depend upon a circuitconnection to be completed when the fuse is blown.

Briefly, in the preferred embodiment of this invention a fuse isemployed having three terminals. The fuse element is connected betweentwo terminals while a spring is connected between the weak section ofthe fuse element and the third terminal. The fuse is so connected thatthe load current flows through the fuse member via two terminals and thethird terminalis connected to a point be tween a glow lamp and aresistor, the latter two devices being connected in series with apotential of sufficient magnitude to fire the glow lamp. When the fusemelts, the spring is freed allowing the potential between resistor andbulb to change and thereby permit full firing potential to be applied tothe lamp.

Other objects of the invention will pointed out in the followingdescription and claims and illustrated in the accompanying drawings,which disclose, by way of example, the principle of the invention andthe best mode that has been contemplated of applying that principle.

In the drawings: v

FIGURE la is a view of the fuse.

FIGURE lb is a view of the fuse element of the fuse in FIGURE 1a.

FIGURE 2a is a circuit diagram of the fuse employed in the indicatingcircuit and a low voltage work circuit.

FIGURES 2b and 2c are circuit arrangements showing suitable potentialsand polarities of the voltage applied.

In FIGURE la a fuse 10 having terminals 12, 14 and 16 is shown. The fuseelement 18 is of a suitable fuse material and is connected across theterminals 12 and 14 with a portion of reduced cross sectional area atthe vertex of the fuse material as shown in FIGURE 1b. A cap or cup 20provides a mounting for the terminals 12 and 14 as well as for the fuseelement and may be of any suitable insulating material. A central orbody portion 22 is composed of some suitable insulating material and isin the form of a hollow cylinder or tube. The body 22 may be made ofglass if it is desired to visually check whether or not the fuse isblown. The terminal 16 may take the form of a cap of suitable conductivematerial to which one end of a spring 24 is attached and the other endof the spring 24 is connected to the fuse portion 18 under tension. Thespring 24 and fuse portion 18 form continuous conducting circuits frompin 12 through cap 16, through terminal 14 through cap 16 and throughterminal 12 through terminal 14. When excessive current flows betweenthe two terminals 12 and 14, 'thefusematerial 18 melts providing aseparation at the point wherethe spring is connected to the element.When the spring is released, its length is decreased to an extentwhereby the circuit connection between terminals 12 and 14 and the cap16 is broken.

In FIGURE 20, a 1.5 volt D.C. work circuit is shown with a load 26connected in series with a 1.5 volt battery 27 and with the fuse element18. Connected to the terminal '16 is a resistor 28 and a glow lamp 30,such as a neon bulb or the like. A pair of 50 volt DC. power supplies 32are in parallel with the resistor 28 and the glow lamp 30 with areference point established by a ground or a chassis connection betweenthe power supplies 32. Before the fuse blows, the terminal 16 is at +1.5volts leaving +515 volts across the neon bulb 30. This voltage is lessthan the firing potential of the glow lamp so it will not fire. When thefuse blows, the point 16 is isolated from the work circuit since thespring 18 retracts and the voltage at the point 16 rises providing afull volts across the lamp 30. The lamp fires and the resistor 28 limitsthe current to some suitable value depending upon the rating of the lamp30.

It is pointed out that for a fuse voltage above 5 volts, the polarity ofthe neon circuit is important in that the voltages should always bearranged where the voltage in the work circuit is to be subtracted fromthe voltage across the neon. In those cases, where the fuse voltagesexceed 5 volts, the polarity must be proper in order that the firingpotential of the bulb 30 is not to be exceeded. FIGURES 2b and 2cillustrate two connections which might be made in those instances wherethe fuse voltages are above 5 volts. However, the selection of voltagesmay vary over wide limits and those shown are merely illustrative andare not to be considered as limiting.

While there have been shown, described and pointed out the fundamentalnovel features of the invention as applied to the preferred embodimentit will be understood that various omissions and substitutions andchanges in the form and details of thedevice illustrated and in itsoperation may be made by those skilled in the art without departing fromthe spirit of the invention. It is the intention, therefore, to belimited as indicated by the scope of the following claims.

What is claimed is:

I. In apparatus for indicating the occurrence of a fault which causes anoverload in a low voltage circuit, a low voltage power source, a loaddevice in series with said power source, a fuse element connected inseries with said load and said low voltage source, a conductivejunction, conductive resilient means under tension coupled from saidfuse element to said junction, an indicator circuit comprising a glowlamp which ionizes at a firing potential connected to said junction, theconnec tion between said conductive junction and said fuse element beingadapted to be opened by said resilient means when said fuse element isdestroyed, a resistor connected to said junction and a voltage sourcegreater than the firing potential of said lamp connected across saidresistor and lamp, and a circuit connection between said low voltagecircuit and said indicating circuit for normally establishing a voltagelevel at said junction less than the firing potential of said lamp.

2. In combination with an electrical supply circuit of low voltage, aglow discharge tube, a current limiting resistor, means connecting saidresistor in series with said glow discharge tube, a fuseably responsiveconnection between said connecting means and said low voltage circuit, asource of potential of a value exceeding the firing potential of saidglow discharge tube across said series resistor and discharge tubecombination, voltage clamping means efiective at said fuseablyresponsive connection for holding the voltage across said lamp belowsaid firing potential, and means responsive to the opening of saidfuseably responsive connection for removing the clamping means from theconnecting means between said resistor and glow discharge tube.

3. In combination, a low voltage circuit having a first voltage source,indicating circuit means including a second source of voltage having amagnitude greater than the magnitude of said first voltage source and avoltage sensitive device operable when the second voltage is applied tosaid device, a fuseably responsive element in said low voltage circuit,and means including a conductive circuit connection between saidfuseably responsive element and said indicating circuit means fornormally prohibiting the entirety of the second voltage from acting onsaid voltage sensitive device, said conductive connection being adaptedto be opened when said fuseably responsive element is desetroyed.

4. A claim in accordance with claim 3 wherein said voltage sensitivedevice is a glow discharge lamp.

5. A claim in accordance with claim 4 wherein said last mentioned meanscomprises a spring of conductive material connected between saidfuseably responsive element and said indicating circuit and undertension in a direction away from said fuseably responsive element.

References Cited in the file of this patent UNITED STATES PATENTS707,894 Andrews Aug. 26, 1902 921,732 Potter May 18, 1909 1,094,030Sprong et a]. Apr. 21, 1914 1,757,753 Thiery et al. May 6, 19302,663,862 De Anthony Dec. 22, 1953 2,683,201 Miller July 6, 19542,717,375 Lubkin Sept. 6, 1955 2,762,884 Van Eyk Sept. 11, 19562,774,960 Podell Dec. 18, 1956

